Freedom of speech is one of the specific rights guaranteed by the First Amendment. It reads “Congress shall make no law … abridging the freedom of speech.”
Freedom of speech was then applied to the states by the Fourteenth Amendment.
While the language of the First Amendment appears to speak in absolute terms (“make no law”), the Supreme Court has debated the meaning of the right for over a century, asking what the authors of the amendment meant by the term “the freedom of speech.” For example, the Court has held that the right provides only limited protection to pornography or indecent speech (both of which it has struggled to define) or to speech intended to incite imminent lawless action and likely to do so. It has provided strongest protection to religious speech and speech on public or political issues. Indeed, many of the most important religious freedom cases have been decided in whole or in part on the basis of freedom of speech. See, e.g., West Virginia State Board of Education v. Barnette (1943).